Method and system for calibrating wireless location systems

ABSTRACT

This invention relates to a method for calibrating a wireless location system (WLS) to enable the system to make highly accurate differential measurements such as time difference of arrival (TDOA) and frequency difference of arrival (FDOA). Calibration is accomplished by transmitting a signal from an unknown location and measuring at each of two receivers the parameter to be calibrated from that part of the received signal reflected or refracted from an object at a known location in the area. A differential measurement error is determined by comparing the expected difference in the parameter measurements with the actual difference in the parameter measurements. The expected difference is known, a priori, based on the locations of the receivers and the location of the object.

FIELD OF THE INVENTION

This invention relates to a method and system for calibrating a wirelesslocation system (WLS) to enable the system to make highly accuratedifferential measurements such as time difference of arrival (TDOA) andfrequency difference of arrival (FDOA).

BACKGROUND OF THE INVENTION

Wireless location systems are becoming increasingly important. Anexample is disclosed in commonly assigned U.S. Pat. No. 5,719,584 toOtto, the disclosure which is incorporated by reference in its entirety.Many wireless location systems use time difference of arrival (TDOA)calculations to determine a set of possible locations of a transmitterof a signal. The location is mathematically determined, as a hyperbolain two dimensions and a hyperboloid in three dimensions, from the knownlocations of two receivers and the difference in the measured time ofarrival (TOA) of the signal at those two receivers. These systems use avariety of methods to measure the TOA of a signal at a receiver. Allshare, however, the common requirement that the clocks must either besynchronized or the offset between the clocks measured and a correctionapplied. The correction may be applied either directly to the clocks ormathematically to the calculations of time differences.

In many wireless location systems an attempt is made to synchronize theclocks at the receivers. A popular method is to use a clock source ateach of the receivers that is synchronized to the global positioningsystem (GPS) transmissions. These systems often use an oscillator withgood short-term stability to drive the clock and apply a correctionbased on the filtered difference between a received GPS timing signal,which has good long term stability, and the clock. These systems requirea GPS receiver and GPS antenna with a clear view of multiple GPSsatellites.

Despite significant recent advances in these systems, theroot-mean-square (RMS) difference in time between two such clocks may beas high as many tens to hundreds of nanoseconds resulting in significanterrors in location estimates, particularly when geometric dilution ofprecision (GDOP) is significant. Although synchronization of the clocksin this manner may reduce TOA measurement errors due to clock offsets,the system must also be carefully designed and calibrated to ensure thatdelays in the receiver processing (both the signal processing chain andthe timing distribution chain) are fixed and properly taken intoaccount.

Some prior art systems use external calibration techniques to correctclock offsets and to correct for other variations in the receivers thatmay introduce TOA (and, therefore TDOA) measurement errors or errors inother measured parameters such as frequency of arrival (FOA). In thesesystems, receivers at known locations measure certain parameters of asignal transmitted by a stationary reference transmitter at a knownlocation. The measured parameters are then communicated to a commonpoint where a processor calculates offsets or adjustments that areeither used to adjust one or both of the receivers or are applied to thetime difference of arrival (TDOA) and/or frequency difference of arrival(FDOA) calculations.

One such prior art wireless location system as disclosed by U.S. Pat.No. 6,184,829 to Stilp, reduces instrumentation error by a calibrationprocess where by multiple wireless transmitters, such as cellulartelephones, are placed at known locations throughout the coverageterritory of the wireless location system. These phones maketransmissions, such as periodic registrations or page responses, in amanner similar to any other phone. Because the location and thetheoretical TDOA values for any pair of receivers are known a priori,the system can determine the error in the TDOA measurements made inconnection with a particular pair of receivers.

In addition, because the phones are in fixed locations and there is noDoppler shift, the theoretical FDOA value is zero. Any measured errorwill be caused by drifts in the oscillators at each of the receivers,changes in the characteristics of analog components (e.g., the antennas,cabling, and filters), and environmental factors. A correction isapplied to the computed TDOA and FDOA values in the digital signalprocessing stages of the system.

These prior art external calibration systems have several limitations.Periodic transmissions must be made by reference transmitters at knownlocations and use system capacity that might otherwise carry normal usertraffic. If a long period elapses between calibration and a subsequentTDOA and/or FDOA measurement, then the calibration may be degraded byoscillator offsets or changes in the receivers because to such things ascomponent parameter shifts are caused by temperature. Although reducingthe interval between reference transmissions improves calibration, it isat the further expense of additional system capacity. Another drawbackis that multipath induced errors in the time of arrival measurementsmade by the receivers during calibration, i.e., when measuring theTOA/TDOA of the reference transmissions, corrupt the TDOA calibrationsince straight-line propagation from the reference transmitter to eachreceiver is presumed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor external calibration of wireless location systems that reduces orremoves TDOA, FDOA and/or other differential measurement errors arisingfrom many sources within the system.

It is another object of the present invention to provide a system andmethod for calibrating wireless location systems that is operable withreference transmitters of unknown location that may be stationary ormobile with unknown vector velocity.

It is yet another object of the present invention to provide a systemand method for calibrating wireless location systems that may use normaluser traffic transmissions from unknown locations by stationary ormobile transmitters of unknown vector velocity as reference transmittersthereby minimizing the system capacity used for calibration and reducingor eliminating degradation in calibration due to system drift betweenthe instant of calibration and the instant of a measurement.

It is yet another object of the present invention to provide a systemand method for calibrating wireless location systems that does notrequire a straight-line propagation path from reference transmitters toreceivers in order to accurately calibrate TDOA or other differencemeasurements.

It is still another object of the present invention to provide a systemand method for calibrating wireless location systems that is useful insystems using fixed, mobile or both fixed and mobile receivers.

In accordance with the present invention, a system and method determinesthe offsets of pairs of receivers used in making TDOA, FDOA and/or otherdifferential measurements of signals. A transmitter at an unknownlocation can be is either stationary or mobile with unknown vectorvelocity. A plurality of fixed or mobile receivers of substantiallyknown or determinable location (and, in the case of moving receiversmaking FOA measurements, of known vector velocity) receive the signalfrom the transmitter via multiple paths due to reflection and refractionof the signal by natural or manmade objects in the vicinity of thetransmitter and/or receivers. The signal arriving at a receiver may ormay not include a straight-line path signal from the transmitter to thatreceiver.

Each receiver measures the TOA and/or FOA or other parameter of at leastone, and in some embodiments several or all, of the path signalsbelieved not to be a straight-line path. Although not necessary forpurposes of calibration, it is preferable that the receiver also measurethe TOA and/or FOA of the straight-line path signal, if present, for usein the course of performing transmitter location and velocitydetermination which may occur coincident with calibration.

A processor is operatively connected to the plural receivers and selectsa stationary natural or manmade object that is believed to havereflected or refracted the signal to each of the plural receivers anddesignates that object as a proxy reference transmitter, also referredto in some instances as proxy receiver because it “receives” a signaland reflects or refracts the “received” signal end thus acts as a “proxyreference transmitter” of the signal. Hereafter, the signal reflected orrefracted by that object to the plural receivers may be referred to as aproxy reference transmission.

The location of the proxy reference transmitter is either stored in adatabase operative with the processor or determinable from informationstored in the database such as, but not limited to, aerial photographicimagery. The processor then determines the differential measurement, foreach combination of receiver pairs receiving the proxy referencetransmission. In the case of FDOA, because the proxy referencetransmitter is a stationary object and the Doppler shift imparted by anymotion of the transmitter relative to the proxy reference transmitter iscommon to the reflected or refracted signal at both receivers, thetheoretical FDOA value is zero. Any measured error will be due to driftsin the oscillators at each of the receivers, changes in thecharacteristics of analog components (e.g., the antennas, cabling, andfilters), and environmental factors.

The processor may also calculate the theoretical TDOA values for eachpair of receivers receiving the proxy reference transmission. Theprocessor determines the expected TDOA from the TOA of the proxyreference transmission at each of the receivers; the locations of thereceivers, which are either known a priori and stored in the database ordeterminable from information stored in the database; and the locationof the proxy reference transmitter, which is either known a priori andstored in the database or determinable from information stored in thedatabase. The processor then either applies corrections to the wirelesslocation system equipment to correct the offsets or applies correctionsto the computed TDOA, FDOA or other parameter values, obtained duringnormal system operation, in the digital signal processing stages of thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIGS. 1 and 2 illustrate a prior art Angle of Arrival and Time ofArrival location determining system.

FIG. 3 is a diagram showing how a proxy receive site having a proxyreceiver, such as a water tower, can be established for determining thelocation of a mobile unit using only one receive site.

FIG. 4 illustrates a diagram similar to FIG. 3, but showing the proxyreceiver as a natural object, such as a hill.

FIG. 5 illustrates how a proxy receiver can be used for calibrating theclocks of first and second receive sites.

FIG. 6 is a diagram illustrating a multipath situation caused by variousbuildings, illustrating the error that occurs when the hyperbola andlocus of points that are established if the proxy receivers areconsidered to be direct line of sight reflectors.

FIG. 7 illustrates the locus of points and error that could occur whenonly an Angle of Arrival analysis is used with proxy receivers.

FIG. 8 is a diagram similar to FIG. 7, but illustrating the locus ofpoints that are developed based on the system and method of the presentinvention, such that the location of a mobile unit can be established.

FIGS. 9 and 10 illustrate flow charts for an image database routine thatcould be used with the present invention.

FIG. 11 is a layout of individual images of a building and texture modelthat could be used with the present invention.

FIG. 12 is another flow chart showing the type of process that could beused with an image database routine shown in FIGS. 9 and 10.

FIGS. 13 and 14 are diagrams, illustrating respective top and isometricviews, where the image database routine determines the three dimensionalaspects of proxy receivers and buildings for height determinations andmore accurate location analysis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

The present invention is advantageous and allows the use of one receiverat a receive site to determine the location of a mobile transmittingunit, such as a transmitting tag for a car or individual wearing thetag, a mobile transceiver, or other mobile unit. The system uses a proxyreceiver (or passive reflector) for Time of Arrival and/or Time ofDifference of Arrival calculations. Throughout the description, the termproxy receiver is used for a reflector/refractor located at a locationcalled a proxy receive site (PRS) and also used to describe any type ofpassive reflector, such as a building, mountain, or hill, water tower,or any other natural or man-made object that would reflect and/orrefract (or diffract) the signal from a transmitting mobile unit orother radio transmitter to a receiver that could be fixed or mobile. Insome instances, the proxy receiver reflects or refracts signals, such asfor calibration, and could be referred to as a reference proxytransmitter. The term mobile unit can refer to any transmitter, fixed ormoving, for purposes of description.

Naturally, the signals can be radio frequency signals, electromagneticsignals, or other signal types known to those skilled in the art. Forexample, some water towers are an excellent reflector of radio frequencysignals, and the water tower itself could be a proxy receiver. A largehill could be a proxy receiver and a building, which not only wouldreflect radio frequency signals, but also diffract/refract radiofrequency signals around a building corner edge.

The present invention is also advantageous because it allows the use ofa look-up table or geographical software imaging database, such as the“RealSite” software as developed by Harris Corporation of Melbourne,Fla., to be used to assist in calculating the mobile location. Thepresent invention is also advantageous because it allows use of themobile transmitting unit for calibrating the clocks of two differentreceivers at two different receive sites, even though the location ofthe unit may be unknown. Two receivers could be used simultaneously todetermine the geolocation of a mobile transmitting unit, even whenmultipath is present, as will be described.

The present invention is advantageous over radio fingerprinting, whichhas limitations and drawbacks. The mobile is always presumed to be atone of the grid points and not at an intermediate location between gridpoints. The elevation of a mobile also changes, of course, such as in abuilding, resulting in errors in the location estimate. The accuracydepends on long transmissions and the motion of the mobile. The use ofnarrow band signals can also limit the ability to discriminate paths andthe extensive calibration is required to generate a fingerprint griddatabase. The embedded mobiles will also have different fingerprints ascompared to exposed units.

FIGS. 1 and 2 illustrate a basic geolocation system and method using anAngle of Arrival (AOA) and Time of Arrival (TOA) system, such asdisclosed in the incorporated by reference '584 patent to Otto, assignedto Harris Corporation of Melbourne, Fla.

With reference to FIG. 1, the geolocation system uses plural fixedreceive sites 30, each connected to a central processing unit 32 throughconventional communication links 34. A target unit (or unit to begeolocated such as a mobile transmitting unit) 36 transmits a radiofrequency signal, which may be an RF signal, an electromagnetic signal,or various types, to plural of the receiving stations.

In operation, the receive sites (or “receiving stations”) 30 eachreceive the signal from the target unit 36 and send a communicationconcerning that signal reception to the central processing unit 32.Depending upon the structure of the system, the central processing unit32 may use the Angle of Arrivals of the signal at the receive sites 30or the Times of Arrival of the signal at the receive sites 30 or otherconventional means to determine the geolocation of the target unit 36.With a proper system design, the central processing system 32 cansimultaneously determine the geolocation of several target units 30located within the receiving range of plural of the receiving stations30.

With reference to FIG. 2, the system requires only two receivingstations RS 30 and RS 40 to determine the geolocation of a target unit36. Each receiving location 30 receives a signal from the target unit 36and determines both the Angle of Arrival and the Time of Arrival of thesignal, which is provided, in turn, to the central processing unit 32through conventional communications links 34. At the central processingunit 32, the difference in Time of Arrival information between twostations may be used to compute a locus of points along a curve 50 atwhich the target unit 36 may exist (i.e., the locus of points from whicha signal would have the determined Time Difference of Arrival betweenthe given two receiving stations 30). The precise point along the curve50 at which the target unit 36 is determined to be located is obtainedfrom the intersection of the curve 50 with the bearing lines (from theAngles of Arrival) AOA1, AOA2 from either one of the receiving stations,RS30 or RS40.

Ideally, the curve 50 and the bearing lines AOA1 and AOA2 all intersectat a single point coincident with the location of the target unit 36. Inpractice, however, errors in the TOA measurements will displace thecurve and/or errors in the AOA measurements will displace one or both ofthe bearing lines such that multiple points of intersection occur, as isshown, thus leading to uncertainty in the location of the mobile unit.

In this type of system in which multiple receiving sites are needed toprovide geolocating (and/or communications) coverage throughout ageographic area, such as in a low power personal communication systems,which cover a wide geographic area, only two receiving stations arerequired for contact with a target unit at any time in order to providecomplete geolocating coverage. If in a given system, the signals from atarget unit are received at more than two receiving stations, theprocessing unit can combine the bearing lines and the curves using anyvectora combination scheme known to those skilled in the art.

Depending upon the local terrain conditions and other factors, thecentral processing unit 32 may: weight the signals from some receivingsites more heavily than others; weight the signals in proportion to ametric of signal quality such as signal-to-noise ratio of the receivedsignals; weight more heavily the positions determined from receivingstations closer to the determined location of the target unit; and/oreliminate some of the bearing lines and/or curves for outlyingestimates, and thus, weighting more heavily the curves determined fromthe TDOA determinations. The central processing unit could determine thegeolocation of the target unit using a moment of inertia calculationbased on the weighted estimates of position. Indeed, the centralprocessing unit may determine the geolocation of the target unit using amoment of inertia calculation based on the weighted estimates ofposition.

There now follows a general overall description of the presentinvention, followed by a detailed description relative to drawing FIGS.3-13. The present invention is advantageous and determines the locationof a transmitter. A transmitter to be located transmits a signal and areceiver receives the signal from a transmitter and measures the Time ofArrival of plural of the multiple path signals from a transmission. Oneor more natural or manmade objects reflect and/or refract portions of asignal from the transmitter toward the receiver.

A database could be used to contain locations of, or information thatmay be used to calculate the locations of, the receiver and one or more,but necessarily all, of the natural or manmade objects. A processor isoperative with the receiver and the database and estimates the locationof the transmitter or determines a set of points representing potentiallocations of the transmitter. This can be accomplished by eitherselecting one object, the location of which is either stored in thedatabase or may be determined from information in the database, as aproxy receiver, or selecting plural objects, the locations of which areeither stored in the database or may be determined from informationstored in the database, as plural proxy receivers.

The location of each proxy receiver is determined from information inthe database. The location of the receiver can be determined frominformation stored in the database and then calculated with astraight-line signal propagation time from each proxy receiver to thereceiver. The Time of Arrival of the signal at each proxy receiver canbe calculated by subtracting the proxy-receiver-to-receiver signalpropagation time for that proxy receiver from the estimated Time ofArrival at the receiver of that path estimated to have been reflected orrefracted from the proxy receiver.

One or more sets of points can be generated with each such setrepresenting potential locations of the transmitter corresponding to thedifference in Time of Arrival of the signal and a proxy receiver fromthe Time of Arrival of the signal at either another proxy signal or thereceiver.

A source or sources of additional information are operatively connectedto the processor and may be necessary or desirable for use by theprocessor by (a) selecting a proxy receiver or plural receivers, (b) asadditional inputs to a multilateration calculation or (c) determiningthe waiting to be applied to points or sets of points and combining themto produce a refined set of points.

The transmitter can be a mobile transmitting unit and the receiver canbe a mobile receiver. The transmitter can also be located indoors, aswell as the receiver. At least one proxy receiver could also be indoors.

The signal could be one of an electromagnetic signal, a radio frequencysignal, an optical signal, or acoustical signal. The signal can begenerated by or at the transmitter, and can be modulated in accordancewith any combination of parameters such as, but not limited to, theidentification number of the transmitter, data stored at thetransmitter, or the status of sensors or switches at the transmitter.The transmitter can also transmit the signal in response to anycombination of states of an internal timer, motion detector, or othersensor or algorithm. The signal can also be transmitted in response toan external command or event such as, but not limited to, a button orswitch closure, or the reception or a trigger or command signal. Thesignal could be the retransmission of a signal received at thetransmitter and modified in any combination of ways such as, but notlimited to, amplitude modulation, phase modulation, frequencytranslation, time shifting, spectral inversion, polarization or anyother such transformation, or modulation as may be used by those skilledin the art. It can also be modified in accordance with any parametersuch as, but not limited to, the identification number of thetransmitter, data stored at the transmitter or the status of sensors orswitches at the transmitter.

The transmitted signal can be a conventional communication systemsignal, but not limited to, cellular telephone, specialized mobileradio, mobile data or personal communications. This transmitted signalcan also have a band width such as, but not limited to, narrow band,wideband, composed of portions with band widths or composed of portionsin non-contiguous frequency bands. The signal can also be a spreadspectrum signal in any modulation format or combination of modulationformats such as, but not limited to, direct sequence, frequency hopping,non-linear frequency, linear frequency (chirp), co-chip key in, codeposition, pulse position or impulse. It can include a straight-line pathsignal from the transmitter to the receiver and can include astraight-line path signal from the transmitter to the receiver.

The at least one object can include a natural landmark or earthelevation, as a proxy receiver, or another man-made structure such as abuilding, portion or a building, water tower, portion or a water tower,communications or utility power pole or portions of a communications orutility power pole.

When a database is used with the present invention, it can containtwo-dimensional or three-dimensional location data. It also can containinformation that may be used to calculate two-dimensional orthree-dimensional locations of an object. The database can include adigitized map, digitized imagery, electromagnetic imagery, radiofrequency imagery, optical imagery or acoustical imagery. The imagerycan be the same frequency band as the transmitted signal or can be in afrequency band different from the transmitted signal.

In another aspect of the present invention, as noted before, there maybe additional information or there may not be additional information forprocessing. This source of additional information could be the receiver,the Angle of Arrival at the receiver or the earliest arriving signalpath, such as the azimuth and elevation. The additional informationcould include the Time of Arrival at the receiver of the earliestarriving signal path or the Angle of Arrival at the receiver of a signalpath estimated by the processor to have been reflected or refracted by aproxy receiver. This Angle of Arrival can include the azimuth andelevation.

The additional information can also include the Angles of Arrival at thereceiver of plural signal paths estimated by the processor to have beenreflected or refracted by plural proxy receivers. The Angle of Arrivalof one or more of the plural reflected or refracted signal paths can beboth azimuth and elevation. The additional information can also include,in any combination, parameters of the received signal or portionsthereof, such as, but not limited to: signal strength, Angle of Arrival,Time of Arrival, multipath profile or roundtrip signal flight time.

The source of additional information could also be the database. Thisadditional information could include information collected from priortransmission by the transmitter. The additional information can includeprior estimates of transmitter location in two or three dimensions.There could also be plural sources of the additional information.

The processor can determine a set of potential locations of thetransmitter corresponding to the difference in Time of Arrival of asignal at a proxy receiver and the Time of Arrival of one other signalpath at the receiver. This other signal path can be the earliestarriving signal path at the receiver. The processor can also determine aset of potential locations of the transmitter corresponding to thedifference in Time of Arrival of a signal path at a first proxy receiverand the Time of Arrival of a signal path at the second proxy receiver.The processor can determine a set of potential locations of thetransmitter corresponding to the intersection of (a) the set ofpotential locations of the transmitter corresponding to the differencein Time of Arrival of a signal path at a proxy receiver as determined bythe Time of Arrival of the signal path reflected or refracted by thatproxy receiver and the Time of Arrival of one other signal path at thereceiver with (b) a set of potential locations of the transmittercorresponding to the Angle of Arrival of the other path at the receiver.

This other signal path can be the earliest arriving path at the receiverand the Angle of Arrival can include both azimuth and elevation.

The processor can also determine a set of potential locations of thetransmitter that is the intersection of (a) the set of potentiallocations of the transmitter corresponding to the difference in Time ofArrival of a first signal path at a first proxy receiver as determinedby the receiver and a second signal path at a second proxy receiver asdetermined by the receiver with (b) the set of potential locations ofthe transmitter corresponding to the Angle of Arrival at the receiver ofa third signal path.

The third signal path can be the earliest arriving signal path at thereceiver. The Angle of Arrival can be determined in both azimuth andelevation. The processor can determine a refined set of potentiallocations of the transmitter by waiting and combining in anycombination: points within a set of points; plural sets of points; andsets of points derived from previous transmissions forpreviously-refined sets of points.

The combined points can include the intersection of plural sets ofpoints or one or more of the points weighted more heavily than others ofthe points. A refined set of points is determined by vector combinationof points in two dimensions. A refined set of points is determined byvector combination of points in three dimensions.

The system can include a plurality of receivers for receiving thesignal, each of which measures the Time of Arrival of plural of themultipath signals from a transmission. The plurality of the receiversincludes two receivers and the processor determines a set of potentiallocations of the transmitter corresponding to the difference in Time ofArrival of a signal at a first proxy receiver as determined from theTime of Arrival of the signal reflected or refracted by the first proxyreceiver at a first receiver and the Time of Arrival of the signal at asecond proxy receiver as determined by the Time of Arrival of the signalreflected or refracted by the second proxy receiver at a secondreceiver.

The processor can determine a set of potential locations of thetransmitter corresponding to the difference in Time of Arrival of asignal at a proxy receiver as determined from the Time of Arrival of thesignal path reflected or refracted by that proxy receiver at a firstreceiver and the Time of Arrival of a signal at a second receiver. Thissecond receiver can be the Time of Arrival of the earliest arrivingsignal path at the second receiver. The earliest arriving signal path atthe second receiver is the straight-line path from the transmitter tothe second receiver.

The processor can determine a refined set of potential locations of thetransmitter by combining plural sets of potential locations of thetransmitter. This set of combined points can be an intersection ofplural sets of points and can be determined by a vector combination ofweighted sets of points in two dimensions. The combined set of pointscan be determined by vector combination of weighted sets of points inthree dimensions.

The receiver can receive a signal from a transmitter and measure theTime of Arrival of plural of the multiple path signals with a signalchannel receiver. It can receive a signal from a transmitter and measurethe Times of Arrival of plural of the multiple path signals in a pluralchannel receiver. One of the plural channels can be operative with adirective antennae pointed in a radial direction along which lies atleast one object that may be selected as a proxy receiver. There can beplural directive antennas, each of which is pointed in a radialdirection along which lies at least one object that may be selected as aproxy receiver and each of which is operative with a separate channel ofthe receiver.

This directive antennae is a sector of a multiple sector antennae andcan be a conventional communications system such as, but not limited to:mobile telephone, specialized mobile radio, or a mobile data. Theplural, but necessarily all, channels of a receiver are each operativewith one or more elements of a plural element antennae such that thoseplural channels include a phased array receiver and can include a lineararray or two-dimensional array that is directive substantially in adirection parallel to the plane of the array. It can also be directivesubstantially in a direction perpendicular to the plane of the array.The plural elements can be operative with the plural channels andinclude a phased array receiver that includes a three-dimensional array.The plural elements can be operative with the plural channels andinclude a phased array receiver that are of the same polarization. Thereceiver can also be plural co-located receivers with substantiallyco-located antennas, including a plural channel receiver that is aphased array receiver.

The Time of Arrival of a signal path can be measured on the linear or avector combination of weighted signals from plural receiver channels.The weights for the channels can be chosen to reduce or null signalsfrom paths other than the path for which the Time of Arrival ismeasured. Polarization of the antennae elements can feed some of thechannels that differ from the polarization of the antennae elementsfeeding other of the channels. This other information can includeinformation about or gathered by the transmitter such as, but notlimited to: configuration (e.g., body worn, vehicle mounted, attached tolarge asset, attached to hand-carried asset); elevation; barometricpressure; temperature; location (e.g., outdoors, indoors, on road); orvelocity. The other information is transmitted by the transmitting unit.

The method and system for calibrating a wireless location system such asreceivers used for locating a transmitter is advantageously set forth. Asignal can be transmitted from an unknown location. A signal is receivedand measured at each of a first and second receiver. An error value isdetermined based on the difference between an expected differencebetween parameter values and the actual difference between measuredparameter values such that the expected difference is determined usinginformation from the database and other sources.

The error value is used to apply corrections to the wireless locationsystem equipment to minimize the errors in subsequent measurementsand/or apply corrections to any prior, concurrent or subsequentmeasurements of the parameter. Throughout this description, the proxyreference transmitter can be a natural or man-made object as describedbefore. It can also be described as the proxy reference receiver orproxy receiver that receives and reflects or refracts signals, thusacting similar to a transmitter. Any error value can also be based onthe difference between an expected frequency difference of arrival andthe actual frequency difference of arrival.

FIG. 3 illustrates a first aspect of the present invention, wherein theprocessor has selected a proxy receiver 60 located at a proxy receivesite (PRS) which is believed to be a reflector/refractor of signals fromthe transmitter to the receiver, as in a typical multipath example. TheTime of Arrival for the proxy receiver equals the Time of Arrival of thereflected or refracted path at the receiver site (RS) minus the proxyreceiver site to receiver site propagation time. Although a water toweris the illustrated proxy receiver 60, it should be understood that aproxy receiver located at a proxy receiver site can be any type ofreflector or refractor such as the flat, reflective side of a building,the edge of a building, a geographic landmark such as a large hillside,a tree in the middle of a prairie, a communications or utility tower,bridge or other reflector/refractor objects as suggested and known bythose skilled in the art.

In the specific illustrated aspect shown in FIG. 3, the receive site(RS) 62 includes a receiver (R₁) and includes an omnidirectional antenna63 that receives a signal from the mobile transmitting unit 64. Theassociated processor 65 is operatively connected to the receiver anddetermines the Time of Arrival of the earliest arriving path signalincident on the omnidirectional antenna using standard processingalgorithms known to those skilled in the art. The computer or otherprocessor of the type known to those skilled in the art 65 could belocated at the receive site or connected via communication lines 66 at adistance from the receive site. In one aspect of the invention, aunidirectional antennae 67 can be pointed directly at the water tower 60and positioned at the receiver site and also operatively connected tothe receiver. The receiver determines the Time of Arrival of thereflected or refracted path signal incident upon the directionalantenna. The processor 65 determines the Time Difference of Arrivalbetween the line of sight signal transmitted to the receive site fromthe mobile transmitting unit and the reflection from the water tower,i.e., the proxy receiver 62. This value is determined based upon theknown location of the water tower as stored in the database.

FIG. 5 illustrates the use of a proxy receiver 72, such as a watertower, for calibrating respective clocks 73, 74 at respective receiver 1(75) and a receiver 2 (76). Calibrated clocks can be criticallyimportant in some applications for determining the location of mobileunits, especially when multipath considerations are taken into account,as in the present invention. Even if the mobile transmitting unit 77 isat an unknown location, this unit can transmit to the proxy receive sitefrom that unknown location against the reflector, acting as a proxyreceiver 72, i.e., the water tower, in the present example. Each receivesite, such as the illustrated receiver 1 and receiver 2, receive thereflection off the water tower acting as the proxy receiver. The receivesites are at fixed, known locations and the distance and angle to thewater tower are known. A central processor 78 is operative with bothreceivers could receive the Time of Arrival and Angle of Arrivalinformation from the two receivers. Through appropriate algorithms andcalculations, the differences are established, and the offset from thetwo clocks 73, 74 can be removed using standard processing algorithms,as known to those skilled in the art. This calibration is advantageousover prior art techniques where known transmitters at known locationsare used to transmit a signal.

FIG. 6 illustrates a situation where multipath is present such as in anurban environment where various buildings, B1-B5 for example, arepresent. A transmitter, such as a mobile transmitting unit 64, islocated behind building B3, which is also positioned central to the fourother buildings B1, B2, B4 and B5. Receiver 1 (Rx₁) is located south ofthe buildings and receiver 2 (Rx₂) is located north of the buildings. Aprocessor is operative with the receivers.

As illustrated, no direct line of sight communications are made from thetransmitter, e.g. mobile transmitting unit 64 to receiver 1 (Rx₁) orreceiver 2 (Rx₂). Instead, the first receiver (Rx₁) receives reflectedfirst and second arriving signals (PR1-1, PR1-2) from buildings B1 andB5, while receiver 2 (Rx₂) receives a signal as a first arriving pathfrom building 2, which could be a as proxy receiver. Thus, the Angle ofeach transmission from a proxy receiver formed by the respectivebuildings and the signal and its Time of Arrival can be determined. Ifthe time difference between the arrival of the signal at the secondreceiver (Rx₂) and the earliest arriving of the two signals at the firstreceiver (Rx₁) is taken, then the hyperbola line drawn at 80 is formed.If the system took the Time Difference of Arrival of the earliest pathat each site (Rx₁ or Rx₂), and the angle from one or the other of thesites, and the two locations are calculated, neither would be correct asillustrated by the two stars 81, 82 shown at A2/TDOA and A1/TDOA. Thefirst star 81 shows angle two with the Time Difference of Arrival andthe second star 82 shows angle one and the Time Difference of Arrival.The hyperbola line drawn at 84 illustrates the correct hyperbola ifthere were only direct line of sight communications. It is evident thatthere are three sites PR1-2, PR2-2, and PR1-1 showing squares for theproxy receivers and the curves are calculated with this ambiguoussolution.

FIG. 7 illustrates a similar situation where an improper line 85 forminga potential locus of points is established when mobile transmitting unit64 is located between buildings B3, B3′, and only Angle of Arrivalconsiderations are used with line of sight values AS2/TD and A1/TD.Improper locations are shown by stars 86 are based on the TimeDifference of Arrival calculations.

In accordance with the present invention, the location, i.e., thelatitude and longitude, of a reflector/refractor located at the proxyreceive site and forming a proxy receiver along each of the arrivingpaths is determined via a look-up table or feature extraction from ageographic image database, such as the software “RealSite,” as developedby Harris Corporation of Melbourne, Fla. The geographic database couldinclude data relating to the natural and man-made features in a specificarea, including data about buildings and natural land formations such ashills.

For example, a database could include information about a specific area,from where a signal emanates, includes a tall building or water tower,being a passive reflector of radio or other signals, and thus act as aproxy receiver. A look-up table could have similar data and the systemprocessor would interrogate and determine from the look-up table thetype of buildings, natural features, etc. from where a signal emanatesto determine what features could be proxy receivers. The use of thegeographic database with a look-up table or the use of featureextraction software is advantageous and allows the system to determineif a direct line of sight path or a reflected and/or refracted path is asource of the signal.

The system could use the feature extraction software or query thelook-up table to determine that the layout shown in FIGS. 6 and 8, whichincludes five buildings, forming the reflections and refractions asillustrated by the signal lines among the buildings. For the exampleshown in FIGS. 6 and 8, a two-dimensional database would be required. Itis possible, however, to also use a three-dimensional database in orderto take into consideration elevation concerns.

In this type of system, where knowledge of reflected/refracted and lineof sight signals could be determined by feature extraction, or thelook-up table used, the location estimate could be calculated usingweighted Time Difference of Arrival curves and Angle of Arrivalbearings. The weights may depend on the number of parameters includingthe number of receive sites that receive a signal, such as receiver 1(Rx₁) and receiver 2 (Rx₂), as illustrated. Other weights could dependon the number of paths received at a site (or in total), as well as thegeometry, i.e., relative locations of the receive sites (Rx₁ and Rx₂),the proxy receive sites (PRS) and the mobile transmitting unit 64. Anestimated signal to noise (S/N) ratio for each path and the locationestimates from prior transmissions could also be used. The locationestimate could also be dependent on the weights from the Angle ofArrival and Time of Arrival estimates.

The process may be iterative. For example, if a mobile unit 64 transmitsa signal that is received at two receive sites, the system couldcalculate three estimates of the location using the receive site Time ofArrival and Angle of Arrival. For example, the following threecalculations could be used: 1) the Angle of Arrival of receive site 1and the Angle of Arrival of receive site 2; 2) the Angle of Arrival fromreceive site 1 and the Time Difference of Arrival of receive site 2minus receive site 2; and 3) the Angle of Arrival for receive site 2 andTime Difference of Arrival for receive site 1 minus receive site 2.

If the variation between the estimate is small, then a weightedcombination could be used. If the variation is large, proxy receivesites could be substituted for one or both receive sites and the systemcould reiterate the calculations. Plural proxy receivers could be usedfor a single Angle of Arrival in this iterative process. The image dataobtained from feature extraction software, such as from RealSite, couldbe used to validate an answer.

For purposes of illustration, a brief description of an example of afeature extraction program that could be used with the presentinvention, such as RealSite, is set forth. Naturally, many differenttypes of feature extraction software are available to one skilled in theart, and can be used in the present invention to determine the variousfeatures that could act as passive reflectors or refractors and be proxyreceivers. Although the present example will be described relative totexture software, radio frequency reflective values could also be usedinstead of texture values as reflected optical effects. The databasecould also be used with two-dimensional or three-dimensional featureimaging. Optical reflectivity can be used for finding building planesurfaces and building edges, which aid in determining the location ofproxy receivers.

Further details of a texture mapping system used for creatingthree-dimensional urban models is disclosed in U.S. patent applicationSer. No. 09/652,118, assigned to the present assignee, the disclosurewhich is hereby incorporated by reference in its entirety. For purposesof description, a high level review of feature extraction using RealSiteis first set forth. This type of feature extraction software can be usedto validate results and find the natural and man-made proxy receiversand can be used in two-dimensional and three-dimensional modes.

RealSite allows the creation of three-dimensional models in texturemapping systems and extends the technology used for terrain texturing tobuilding texture by applying clip mapping technology to urban scenes. Itcan be used to determine optical reflectivity values and even radiofrequency reflectivity for determining proxy receivers and determiningthe latitude and longitude of such proxy sites.

It is possible to construct a single image of a building from manyimages that are required to paint all the sites. Building site imagescan fit into a composite image of minimum dimension, including rotationsand intelligent arrangements. Any associated building vertex texturecoordinates can be scaled and translated to match new composite images.The building images can be arranged in a large “clip map” image,preserving the horizontal relationships of the buildings. If thehorizontal relationships cannot be accurately preserved, a “clip grid”middle layer can be constructed, which can be used by the displaysoftware to accurately determine the clip map center.

At its highest level, the system creates a packed rectangle of texturesfor each of a plurality of three-dimensional objects corresponding tobuildings to be modeled for a geographic site. The system spatiallyarranges the packed rectangle of textures in a correct position within asite model clip map image. The texture mapping system can be used with acomputer graphics program run on a host or client computer having anOpenGL application programming interface. The location of a clip centerwith respect to a particular x,y location for the site model clip mapimage can be determined by looking up values within a look-up table,which can be built by interrogating the vertices of all building polygonfaces for corresponding texture coordinates. Each texture coordinate canbe inserted into the look-up table based on the corresponding polygonface vertex coordinate.

In these types of systems, the graphics hardware architecture could behidden by the graphics API (Application Programming Interface). Althoughdifferent programming interfaces could be used, a preferred applicationprogramming interface is an industry standard API such as OpenGL, whichprovides a common interface to graphics functionality on a variety ofhardware platforms. It also provides a uniform interface to the texturemapping capability supported by the system architecture.

OpenGL allows a texture map to be represented as a rectangular pixelarray with power-of-two dimensions, i.e., 2^(m)×2^(n). To increaserendering speed, some graphics accelerators use pre-computed reducedresolution versions of the texture map to speed up the interpolationbetween sampled pixels. The reduced resolution image pyramid layers arereferred to as MIPmaps by those skilled in the art. MIPmaps increase theamount of storage each texture occupies by 33%.

OpenGL can automatically compute the MIPmaps for a texture, or they canbe supplied by the application. When a textured polygon is rendered,OpenGL loads the texture and its MIPmap pyramid into the texture cache.This can be very inefficient if the polygon has a large texture, buthappens to be far away in the current view such that it only occupies afew pixels on the screen. This is especially applicable when there aremany such polygons.

Further details of OpenGL programming are found in Neider, Davis andWoo, OpenGL Programming Guide, Addison-Wesley, Reading, Mass., 1993,Chapter 9, the Guide disclosure which is hereby incorporated byreference in its entirety.

Clip texturing can also be used, which improves rendering performance byreducing the demands on any limited texture cache. Clip texturing canavoid the size limitations that limit normal MIPmaps by clipping thesize of each level of a MIPmap texture to a fixed area clip region.

Further details for programming and using clip texturing can be found inSilicon Graphics, IRIS Performer Programmer's Guide, Silicon Graphics,Chapter 10: Clip Textures, the Programmer's Guide, which is herebyincorporated by reference in its entirety.

IRIS Performer is a three-dimensional graphics and visual simulationapplication programming interface that lies on top of OpenGL. Itprovides support for clip texturing that explicitly manipulates theunderlying OpenGL texture mapping mechanism to achieve optimization. Italso takes advantage of special hardware extensions on some platforms.Typically, the extensions are accessible through OpenGL as platformspecific (non-portable) features.

In particular, IRIS Performer allows an application to specify the sizeof the clip region, and move the clip region center. IRIS Performer alsoefficiently manages any multi-level paging of texture data from slowersecondary storage to system RAM to the texture cache as the applicationadjusts the clip center.

Preparing a clip texture for a terrain surface (DEM) and applying it canbe a straightforward software routine in texture mapping applications,as known to those skilled in the art. An image or an image mosaic isorthorectified and projected onto the terrain elevation surface. Thissingle, potentially very large, texture is contiguous and mapsmonotonically onto the elevation surface with a simple verticalprojection.

Clip texturing an urban model, however, is less straightforward of asoftware application. Orthorectified imagery does not always map ontovertical building faces properly. There is no projection direction thatwill map all the building faces. The building textures comprise a set ofnon-contiguous images that cannot easily be combined into a monotoniccontiguous mosaic. This problem is especially apparent in an urban modelhaving a number of three-dimensional objects, typically representingbuildings and similar vertical structures. It has been found that it isnot necessary to combine contiguous images into a monotonic contiguousmosaic. It has been found that sufficient results are achieved byarranging the individual face textures so that spatial locality ismaintained.

FIG. 9 illustrates a high level flow chart illustrating basic aspects ofa texture application software model, such as could be used inconjunction with the present invention as related to geolocating amobile transmitting unit. The system creates a packed rectangle oftextures for each building (block 100). The program assumes that thelocality is high enough in this region that the actual arrangement doesnot matter. The packed textures are arranged spatially (block 102). Thespatial arrangement matters at this point, and there are some trade-offsbetween rearranging things and the clip region size. A clip grid look-uptable, however, is used to overcome some of the locality limitations(block 104), as explained in detail below.

Referring now to FIG. 10, a more detailed flow chart sets forth thesequence of steps. A composite building texture map (CBTM) is created(block 110). Because of tiling strategies used later in a site modelclip mapping process, all images that are used to texture one buildingare collected from different viewpoints and are packed into a singlerectangular composite building texture map. To help reduce the area ofpixels included in the CBTM, individual images (and texture mapcoordinates) are rotated (block 112) to minimize the rectangular areainside the texture map actually supporting textured polygons. Afterrotation, extra pixels outside the rectangular footprint are cropped off(block 114).

Once the individual images are pre-processed, image sizes for eachcontributing image are loaded into memory (block 116). These dimensionsare sorted by area and image length (block 118). A new image size havingthe smallest area, with the smallest perimeter, is calculated, whichwill contain all the building's individual textures (block 120). Theindividual building textures are efficiently packed into the new imageby tiling them alternately from left to right and vice versa, such thatthe unused space in the square is minimized (block 122).

FIG. 11 illustrates an example of a layout showing individual images ofa building in the composite building texture map. This is accomplishedby an exhaustive search as described to calculate the smallest imagedimensions describing each building.

A site model clip map image is next created. Because each compositebuilding texture map (CBTM) is as small as possible, placing each onespatially correct in a large clip map is realizable. Initially, eachcomposite building texture map is placed in its correct spatial positionin a large site model clip map (block 124). A scale parameter is used toinitially space buildings at further distances from each other whilemaintaining relative spatial relations (block 126). Then each compositebuilding texture map is checked for overlap against the other compositebuilding texture maps in the site model clip map (block 128). The sitemodel clip map is expanded from top right to bottom left until nooverlap remains (block 130). For models with tall buildings, a largerpositive scale parameter may be used to allow for the increasedlikelihood of overlap. All texture map coordinates are scaled andtranslated to their new positions in the site model clip map image.

Referring now to FIG. 12, a flow chart illustrates the basic operationthat can be used to process and display building clip texturescorrectly. A clip map clip grid look-up table is used to overcome theselimitations and pinpoint the exact location of where the clip centeroptimally should be located with respect to a particular x,y location.To build the table, the vertices of all the building polygon faces areinterrogated for their corresponding texture coordinates (block 150).Each texture coordinate is inserted into a look-up table based on itscorresponding polygon face vertex coordinates (block 152).

A clip center or point in the clip map is used to define the location ofthe highest resolution imagery within the clip map (block 154).Determining this center for a terrain surface clip map is actuallyachievable with little system complexity because a single clip texturemaps contiguously onto the terrain elevation surface, so the cameracoordinates are appropriate. The site model clip map has a clip centerof its own and is processed according to its relative size and positionon the terrain surface (block 156). The site model clip map, however,does introduce some locality limitations resulting from tall buildingsor closely organized buildings. This necessitates the use of anadditional look-up table to compensate for the site model clip map'slack of complete spatial coherence. The purpose of the clip grid is tomap 3D spatial coordinates to clip center locations in the spatiallyincoherent clip map.

The clip grid look-up table indices are calculated using a x,y scenelocation (the camera position) (block 158). If the terrain clip map andsite model clip map are different sizes, a scale factor is introduced tonormalize x,y scene location for the site model clip map (block 160). Ithas been found that with sufficient design and advances in thedevelopment of the spatial correctness of the building clip map, theneed for the clip grid look-up table can be eliminated in up to 95% ofthe cases.

It is also possible to extend the algorithm and use multiple site modelclip maps. Using many smaller clip maps rather than one large clip mapmay prove to be a useful approach if clip maps of various resolutionsare desired or if the paging in and out of clip maps from process spaceis achievable. However, it requires the maintenance of multiple clipcenters and the overhead of multiple clip map pyramids.

Using the image database, such as the RealSite database and associatedsoftware, or a look-up table, if available, it is possible to determineif a proxy receiver also has a certain elevation as a reflector, asshown in FIGS. 13 and 14. FIG. 13 shows a plan view of a buildinglayout, where a first building B10 is located in front of a secondbuilding B11. First building B10 has a lower elevation than buildingB11. The transmitter or mobile transmitting unit 64 is located behindthe smaller, first building B10 and reflects its signal off the taller,second building B11 to two receive sites having receivers Rx₁ and Rx₂.Line 92 is representative of a locus of points that are representativeof the improper proxy receiver designation due to lack of sufficientknowledge of the elevation of respective buildings B10 and B11 and thuscausing an improper designation. Dots 94 represent possible locationsbased on that data. Line 95 represents the hyperbola with the properproxy receiver designation and dots 96 represent the possible locations,indicting a greater accuracy. The image database can be used todetermine the proper hyperbola and angle to determine the TimeDifference of Arrival among associated signals and the proper reflectors(or refractors) and determine the approximate location of the mobiletransmitting unit.

As noted above, a Time Difference of Arrival (TDOA) system and not anAngle of Arrival system could use highly directive antennas pointed atselected proxy receivers, such as a water tower, a huge building, a hillor other objects having the requisite signal reflectivity and acting asa proxy receiver. Also, the elevation Angle of Arrival measurementscould be used to determine the height of the proxy receiverreflector/refractor. As a result, the line of sight mobile elevationinduced slant range errors could be reduced. The proxy receiverreflector/refractor height data would allow slant range calculationsfrom the proxy receiver and would allow the estimation of mobileelevation if a person held a mobile transmitting unit, such as atransmitting tag, personal device such as a mobile tracking device, orother transmitter device.

If one of the receive sites has a direct path, and a proxy receiver isused for the other site, the Time Difference of Arrival error will bereduced unless the proxy receiver happens to lie on the correct bearingto the mobile transmitting unit. If the proxy receiver is on the correctbearing to the mobile transmitting unit, the Time Difference of Arrivalerror would not change, but the hyperbola will rotate so that it issymmetric about the line between the proxy receiver and the otherreceive site, as shown in FIG. 13.

If two sites measure the Time of Arrival of a signal from the same proxyreceiver, and the calculated Time of Arrival at the proxy receiverdiffers by the clock offset between the two receive sites, the clockerror could then be removed as explained before. This can occur evenwhen the transmitter is of an unknown location. This type of system isadvantageous over a transmitter at a known location, which is used todetermine clock offsets if the clock offset is calculated within a fewsamples of all the measured Time of Arrivals and does not represent anyadditional capacity use.

Beam forming could also be used to point out a specific proxy receiver.Many different types of antennae, as known to those skilled in the art,could be used for beam forming. It is presumed that nulling of pathswould be used to remove impacts of other paths on the arriving path. Fora four-element system, the system could place nulls on three otherpaths. It is possible that a user of the system desires to null thelargest or may want to null those most closely aligned in time with thepath of interest.

The Time of Arrival can be measured by any means known to those skilledin the art, including cross correlating signals from two sites.Naturally, the proxy receiver as a passive reflector could be the earth,and could require the elevation Angle of Arrival. Active repeaters couldbe used versus passive repeaters, but there would have to be somecalculation to account for repeater delay.

There is no requirement that the a proxy receiver be a reflector.Another radio receive site or a transmitter as a mobile transmittingunit could act as a proxy receiver. In addition, the system and methodof the present invention could be used to determine the location of amobile reflector as a proxy receiver. For example, the technique couldbe used as a passive radar by an aircraft to determine the location ofanother aircraft.

The first aircraft would measure the Time of Arrival of a signal from atransmitter with a known, but not necessarily fixed location. Theaircraft would also measure the Time of Arrival and Angle of Arrival forthe same signal reflected by the target, which could be an aircraft. Anybeam steering and nulling could be used to improve the ability to detectreflected signals and measure the direction of the arrival. Using theknown location of a first aircraft and the transmitter and the directionof arrival of the reflected signal, the location of an aircraft could becalculated.

A transmitter could be one operating cooperatively with the firstaircraft or an unwilling third party, such as an FM radio station oreven a satellite transponder. By measuring the Time of Arrival anddirection of arrival for a signal reflected from the ground and the Timeof Arrival and the direction of arrival of a direct pass signal, bothemanating from a second aircraft, a first aircraft may determine thelocation of a second aircraft.

The advantages of the system and method of the present invention aremanifest because location can be estimated from a single site if morethan one path is received and an appropriate line of sight or databaseinformation is known concerning the geographically oriented proxyreceivers. Accuracy is improved with time base errors being eliminatedfor Time Difference of Arrival calculations involving the receive siteand associated proxy receivers. Time of Arrivals can be measuredrelative to the same clock. A significant portion of the mulipathinduced Time of Arrival error is reduced in non-line-of-sightsituations. Multiple solution sets can provide additional informationand allow for weighting and averaging. The imagery from a database canbe used to apply corrections.

It is also possible to obtain mobile elevation estimates and mobilelocations that are not restricted to a grid. The system is robustagainst changing multipath because of mobile elevation changes.Extensive calibration is not required. A reflector database does nothave to be extensive and could be generated from photographs such as theRealSite database. It could be useful for any system with Angle ofArrival or Time of Arrival systems, i.e. cell systems.

This application is related to copending patent applications entitled,“SYSTEM AND METHOD FOR DETERMINING THE LOCATION OF A TRANSMITTER USINGPASSIVE REFLECTORS OR REFRACTORS AS PROXY RECEIVERS” and “SYSTEM ANDMETHOD FOR DETERMINING THE LOCATION OF A TRANSMITTER USING PASSIVEREFLECTORS OR REFRACTORS AS PROXY RECEIVERS AND USING DATABASEQUERYING,” which are filed on the same date and by the same assignee andinventors, the disclosures which are hereby incorporated by reference.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that themodifications and embodiments are intended to be included within thescope of the dependent claims.

That which is claimed is:
 1. A system for calibrating receivers used forlocating a transmitter comprising: a transmitter at an unknown locationthat transmits a signal; a proxy receiver that reflects and/or refractsthe signal; a plurality of spatially oriented receivers that receive thesignal from the proxy receiver, each receiver having a clock fordetermining Time of Arrival, Time Difference of Arrival, or FrequencyDifference of Arrival of signals, wherein the clocks are calibratedbased on the clock offset.
 2. A system according to claim 1, and furthercomprising a processor associated with each receiver for calculatingTime of Arrival, Time Difference of Arrival, or Frequency Difference ofArrival and clock offset.
 3. A system according to claim 1, wherein saidtransmitter comprises a mobile unit.
 4. A system according to claim 1,wherein said signal comprises a radio frequency signal.
 5. A systemaccording to claim 1, wherein said proxy receiver comprises a naturallandmark.
 6. A system according to claim 5, wherein said proxy receivercomprises an earth elevation.
 7. A system according to claim 1, whereinsaid proxy receiver comprises a man-made structure.
 8. A systemaccording to claim 7, wherein said proxy receiver comprises a building.9. A system according to claim 1, wherein said plurality of receiverscomprise two receivers.
 10. A system according to claim 1, wherein eachclock comprises a stable local clock.
 11. A system for calibratingreceivers used for locating a transmitter determination comprising: atransmitter at an unknown location that transmits a signal; a proxyreceiver that reflects and/or refracts the signal; a plurality ofspatially oriented receivers each having an antenna directed at theproxy receiver for receiving the signal from the proxy receiver, eachreceiver having a clock for determining Time of Arrival, Time Differenceof Arrival, or Frequency Difference of Arrival of signals, wherein theclocks are calibrated based on the clock offset.
 12. A system accordingto claim 11, wherein each antenna comprises a focused beam antenna. 13.A system according to claim 11, and further comprising a processorassociated with each receiver for calculating at least one of Time ofArrival, Time Difference of Arrival, or Frequency Difference of Arrivaland clock offset.
 14. A system according to claim 11, wherein saidtransmitter comprises a mobile unit.
 15. A system according to claim 11,wherein said signal comprises a radio frequency signal.
 16. A systemaccording to claim 11, wherein said proxy receiver comprises a naturallandmark.
 17. A system according to claim 16, wherein said proxyreceiver comprises an earth elevation.
 18. A system according to claim11, wherein said proxy receiver comprises a man-made structure.
 19. Asystem according to claim 18, wherein said proxy receiver comprises abuilding.
 20. A system according to claim 11, wherein said plurality ofreceivers comprise two receivers.
 21. A system according to claim 11,wherein each clock comprises a stable local clock.
 22. A method ofcalibrating spatially oriented receivers used in a locationdetermination system comprising the steps of: transmitting a signal froman unknown location; receiving within each spatially oriented receiverthe signal after having reflected and/or refracted from a proxy receiverhaving a known location, each receiver having a clock for determining atleast Time of Arrival, Time Difference of Arrival, or FrequencyDifference of Arrival of signals; calculating the Time of Arrival, TimeDifference of Arrival, or Frequency Difference of Arrival for the signalat each receiver; determining the clock offset between the plurality ofreceivers based on the Time of Arrival, Time Difference of Arrival, orFrequency Difference of Arrival of the signal within each receiver; andcalibrating the clocks based on the clock offset.
 23. A method accordingto claim 22, and further comprising the step of transmitting the signalfrom a mobile unit.
 24. A method according to claim 22, wherein saidsignal comprises a radio frequency signal.
 25. A method according toclaim 22, wherein said proxy receiver comprises a natural landmark. 26.A method according to claim 25, wherein said proxy receiver comprises anearth elevation.
 27. A method according to claim 22, wherein said proxyreceiver comprises a man-made structure.
 28. A method according to claim27, wherein said proxy receiver comprises a building.
 29. A methodaccording to claim 27, wherein each clock comprises a stable localclock.
 30. A method according to claim 27, wherein said plurality ofreceivers comprise two receivers.
 31. A method of calibrating spatiallyoriented receivers used in a location determination system comprisingthe steps of: transmitting a signal from an unknown location; reflectingand/or refracting the signal from a proxy receiver having a knownlocation; directing an antenna of each receiver at the proxy receiverfor receiving the signal within each spatially oriented receiver;calculating the Time of Arrival, Time Difference of Arrival, orFrequency Difference of Arrival for the signal at each receiver;determining the clock offset between the plurality of receivers; andcalibrating the clocks based on the clock offset.
 32. A method accordingto claim 31, wherein each antenna comprises a narrow beam antenna.
 33. Amethod according to claim 31, and further comprising the steps oftransmitting the signal from a mobile transmitter unit.
 34. A methodaccording to claim 31, wherein said signal comprises a radio frequencysignal.
 35. A method according to claim 31, wherein said proxy receivercomprises a natural landmark.
 36. A method according to claim 35,wherein said proxy receiver comprises an earth elevation.
 37. A methodaccording to claim 31, wherein said proxy receiver comprises a man-madestructure.
 38. A method according to claim 35, wherein said proxyreceiver comprises a building.
 39. A method according to claim 31,wherein each clock comprises a stable local clock.
 40. A methodaccording to claim 31, wherein said plurality of receiver s comprise tworeceivers.